This invention relates to a method and apparatus for measuring the light absorbance by a fluid. In analysis of very small quantities of liquids, it has been recognized that the physical conditioning of the fluid must be done very carefully. Thus, for example, in the field of liquid chromatography wherein very small, continuously-flowing streams of liquid are measured, care is taken to minimize mechanical and thermal disturbance of the liquid stream between the chromatographic column and analytical apparatus in which the liquid stream from the column is to be continuously analyzed. The primary objective is to present, to a transparent sample cell, the precise sequence of changing liquid composition that leaves the chromatography column.
The rationale and particulars of such apparatus are described in the art. For example, see U.S. Pat. No. 3,674,373 to Waters, Hutchins and Abrahams which involves a refractometer particularly well adapted to receive such a liquid stream. In general, the approach is to minimize the conduit path through which the liquid to be analyzed must travel and to provide a maximum thermal-conditioning of the liquid within such a minimized path. This generally illustrates the art-recognized importance of careful handling of sample liquid between its point of origin and the sample cell in which it is to be subjected to analysis, usually analysis which measures an effect of the sample liquid stream on some radiation directed into a flow-cell through which the stream passes.
Investigators have also realized that some attention must be given to the physical condition of the fluid even after it enters the flow-cell. Consequently, flow-cells have been made ever smaller to avoid mixing and peak-spreading effects and, in some cases, a positive thermal equilibration of the cell with the liquid has been sought in order to avoid light-shimmering effects along the cell walls. Moreover, the cells are usually positioned with outlets so placed that any entrained gas bubbles tend to be carried upwardly out of the cell.
U.S. Pat. No. 3,792,929, to Alpert, it has been noted, seems to disclose a conical sample-holding cell. The patent is related to static-sample devices and in no way involves fluid lenses of any type. The apparent and relative dimensions of the Alpert cell would not allow its effective use in most continuous-flow monitoring systems such as are encountered in liquid chromatographic work and the like.
U.S. Pat. No. 4,011,451 to Nelson discloses a photometer utilizing a generally conical sample cell with the smaller end of the cell being nearer the light source. The photometer substantially eliminates spurious radiation signals generated by a lens type effect caused by liquids of different refractive index that leads to undesirable light absorbance by the sample cell walls. The apparatus described in U.S. Pat. No. 4,011,451 utilizes a primary light source having a wavelength of 253.7 nanometers.
Presently there is a need for photometric apparatus which utilizes a shorter ultraviolet light wavelength than that utilized by prior art photometric apparatus to permit detection of a wide variety of biological compounds. A suitable shorter primary light source has a wavelength of 214 nanometers produced from a zinc lamp or a wavelength of 229 nanometers from a cadmium lamp. However, the shorter wavelength light has lower energy than the 253.7 nanometer light utilized in presently available photometric apparatus. This causes a higher noise to signal ratio which leads to errors in measurement. Furthermore, it would be desirable to provide a photometric process and apparatus which permits utilizing short wavelength ultraviolet light.